In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), and Ultra Mobile Broadband (UMB), just to mention a few.
Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to evolve the WCDMA standard towards the fourth generation of mobile telecommunication networks. In comparisons with WCDMA, LTE provides increased capacity, much higher data peak rates and significantly improved latency numbers. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing (OFDM) is used in a downlink (DL) transmission from a radio base station to a user equipment. Single Carrier-Frequency Domain Multiple Access (SC-FDMA) is used in an uplink (UL) transmission from the user equipment to the radio base station. Services in LTE are supported in the packet switched domain. The SC-FDMA used in the uplink is also referred to as Discrete Fourier Transform Spread (DFTS)-OFDM.
The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix (cp), which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies f or subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes, #0-#9, each with a Tsubframe=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station or radio base station transmits control information about to which user equipments or terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols used for control signaling is illustrated in FIG. 3 and denoted as control region. The resource elements used for control signaling are indicated with wave-formed lines and resource elements used for reference symbols are indicated with diagonal lines. Frequencies f or subcarriers are defined along an z-axis and symbols are defined along an x-axis.
LTE uses hybrid-Automatic Repeat Request (ARQ), where, after receiving downlink data in a subframe, the user equipment attempts to decode it and reports to the radio base station using uplink control signaling whether the decoding was successful by sending an Acknowledgement (ACK) if successful decoding or a “non Acknowledgement” (NACK) if not successful decoding. In case of an unsuccessful decoding attempt, the radio base station may retransmit the erroneous data.
Uplink control signaling from the user equipment or terminal to the base station or radio base station comprises
hybrid-ARQ acknowledgements for received downlink data;
user equipment or terminal reports related to the downlink channel conditions, used as assistance for the downlink scheduling;
scheduling requests, indicating that a user equipment or terminal needs uplink resources for uplink data transmissions.
Uplink control information may be transmitted in two different ways:
on the Physical Uplink shared Channel (PUSCH). If the user equipment or terminal has been assigned resources for data transmission in the current subframe, uplink control information, including hybrid-ARQ acknowledgements, is transmitted together with data on the PUSCH.
on the Physical Uplink Control Channel (PUCCH). If the user equipment or terminal has not been assigned resources for data transmission in the current subframe, uplink control information is transmitted separately on PUCCH, using resource blocks specifically assigned for that purpose.
Herein the focus is on the latter case, i.e. where Layer1/Layer2 (L1/L2) control information, exemplified by channel-status reports, hybrid-ARQ acknowledgements, and scheduling requests, is transmitted in uplink resources, i.e. in the resource blocks, specifically assigned for uplink L1/L2 control information on the Physical Uplink Control Channel (PUCCH). Layer 1 comprises a physical layer and Layer 2 comprises the data link layer. As illustrated in FIG. 4, PUCCH resources 41, 42 are located at the edges of the total available cell uplink system bandwidth. Each such resource comprises twelve “subcarriers”, i.e. it comprises one resource block, within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources are frequency hopping on the slot boundary, as illustrated by the arrow, i.e. within a subframe there is one “resource” 41 comprising 12 subcarriers at the upper part of the spectrum within a first slot of the subframe and an equally sized resource 42 at the lower part of the spectrum during a second slot of the subframe or vice versa. If more resources are needed for the uplink L1/L2 control signaling, e.g. in case of very large overall transmission bandwidth supporting a large number of users, additional resource blocks may be assigned next to the previously assigned resource blocks. Frequencies f or subcarriers are defined along an z-axis and symbols are defined along an x-axis.
The reasons for locating the PUCCH resources at the edges of the overall available spectrum are:
Together with the frequency hopping described above, the location of the PUCCH resources at the edges of the overall available spectrum maximizes the frequency diversity experienced by the control signaling.
Assigning uplink resources for the PUCCH at other positions within the spectrum, i.e. not at the edges, would have fragmented the uplink spectrum, making it impossible to assign very wide transmission bandwidths to single mobile user equipment or terminal and still retain the single-carrier property of the uplink transmission.
The bandwidth of one resource block during one subframe is too large for the control signaling needs of a single user equipment or terminal. Therefore, to efficiently exploit the resources set aside for control signaling, multiple user equipments or terminals may share the same resource block. This is done by assigning the different user equipments or terminals different orthogonal phase rotations of a cell-specific length-12 frequency-domain sequence.
The resource used by a PUCCH is therefore not only specified in the time-frequency domain by the resource-block pair, but also by the phase rotation applied. Similarly to the case of reference signals, there are up to twelve different phase rotations specified, providing up to twelve different orthogonal sequences from each cell-specific sequence. However, in the case of frequency-selective channels, not all the twelve phase rotations may be used if orthogonality is to be retained. Typically, up to six rotations are considered usable in a cell.
As mentioned above, uplink L1/L2 control signaling includes hybrid-ARQ acknowledgements, channel-status reports and scheduling requests. Different combinations of these types of messages are possible, using one of two available PUCCH formats, capable of carrying different number of bits.
PUCCH format 1. There are actually three formats, 1, 1a, and 1b in the LTE specifications, although herein they are all referred to as format 1 for simplicity.
PUCCH format 1 is used for hybrid-ARQ acknowledgements and scheduling requests. It is capable of carrying up to two information bits in addition to Discontinuous Transmission (DTX). If no information transmission was detected in the downlink, no acknowledgement is generated, also known as DTX. Hence, there are 3 or 5 different combinations, depending on whether MIMO was used on the downlink or not. This is illustrated in FIG. 5. In col 51 the combination index is denoted, in col 52 the ARQ information sent when no MIMO is used is disclosed, and in col 53 the ARQ information when MIMO is used when a first transport block and a second transport block are received is shown.
PUCCH format 1 uses the same structure in the two slots of a subframe, as illustrated in FIG. 6. For transmission of a hybrid-ARQ acknowledgement (ACK), the single hybrid-ARQ acknowledgement bit is used to generate a Binary Phase-Shift Keying (BPSK) symbol, in case of downlink spatial multiplexing the two acknowledgement bits are used to generate a Quadrature Phase Shift Keying (QPSK) symbol. For a scheduling request, on the other hand, the BPSK/QPSK symbol is replaced by a constellation point treated as negative acknowledgement at the radio base station or evolved NodeB (eNodeB). Each BPSK/QPSK symbol is multiplied with a length-12 phase rotated sequence. These are then weighted with a length-4 sequence before transformed in an IFFT process. Phase shifts vary on SC-FDMA or DFTS-OFDM symbol level. The reference symbols (RS) are weighted with a length-3 sequence. The modulation symbol is then used to generate the signal to be transmitted in each of the two PUCCH slots. BPSK modulation symbols, QPSK modulation symbols, and complex valued modulation symbols are examples of modulation symbols.
For PUCCH format 2, there are also three variants in the LTE specifications, formats 2, 2a and 2b, where the last two formats are used for simultaneous transmission of hybrid-ARQ acknowledgements as discussed later in this section. However, for simplicity, they are all referred to as format 2 herein.
Channel-status reports are used to provide the radio base station or eNodeB with an estimate of the channel properties at the user equipment or terminal in order to aid channel-dependent scheduling. A channel-status report comprises multiple bits per subframe. PUCCH format 1, which is capable of at most two bits of information per subframe, can obviously not be used for this purpose. Transmission of channel-status reports on the PUCCH is instead handled by PUCCH format 2, which is capable of multiple information bits per subframe.
PUCCH format 2, illustrated for normal cyclic prefix in FIG. 7, is based on a phase rotation of the same cell-specific sequence as format 1, i.e. lenghth-12 phase rotated sequence that is varying per SC-FDMA or DFTS-OFDM symbol. The information bits are block coded, QPSK modulated, each QPSK symbol b0-b9 from the coding is multiplied by the phase rotated length-12 sequence and all SC-FDMA or DFTS-OFDM symbols are finally IFFT processed before transmitted.
In order to meet the upcoming International Mobile Telecommunications (IMT)-Advanced requirements, 3GPP is currently standardizing LTE Release 10 also known as LTE-Advanced. One property of Release 10 is the support of bandwidths larger than 20 MHz while still providing backwards compatibility with Release 8. This is achieved by aggregating multiple component carriers, each of which can be Release 8 compatible, to form a larger overall bandwidth to a Release 10 user equipment. This is illustrated in FIG. 8, where five 20 MHz are aggregated into 100 MHz.
In essence, each of the component carriers in FIG. 8 is separately processed. For example, hybrid ARQ is operated separately on each component carrier, as illustrated in FIG. 9. For the operation of hybrid-ARQ, acknowledgements informing the transmitter on whether the reception of a transport block was successful or not is required. A straightforward way of realizing this is to transmit multiple acknowledgement messages, one per component carrier. In case of spatial multiplexing, an acknowledgement message would correspond to two bits as there are two transport blocks on a component carrier in this case already in the first release of LTE. In absence of spatial multiplexing, an acknowledgement message is a single bit as there is only a single transport block per component carrier. Each flow F1-Fi illustrates a data flow to the same user. Radio Link control (RLC) for each received data flow is performed on the RLC layer. In the Medium Access Control (MAC) layer MAC multiplexing and HARQ processing is performed on the data flow. In the physical (PHY) layer the coding and OFDM modulation of the data flow is performed.
Transmitting multiple hybrid-ARQ acknowledgement messages, one per component carrier, may in some situations be troublesome. If the current LTE Frequency Division Multiplex (FDM) uplink control signaling structures are to be reused, at most two bits of information may be sent back to the radio base station or eNodeB using PUCCH format 1.
One possibility is to bundle multiple acknowledgement bits into a single message. For example, ACK could be signaled only if all transport blocks on all component carriers are correctly received in a given subframe, otherwise a NACK is fed back. A drawback of this is that some transport blocks might be retransmitted even if they were correctly received, which could reduce performance of the system.
Introducing a multi-bit hybrid-ARQ acknowledgement format is an alternative solution. However, in case of multiple downlink component carriers, the number of acknowledgement bits in the uplink may become quite large. For example, with five component carriers, each using MIMO, there are 55 different combinations, keeping in mind that the DTX is preferably accounted for as well, requiring at least log2(55)≈11.6 bits. The situation can get even worse in Time Division Duplex (TDD), where multiple downlink subframes may need to be acknowledged in a single uplink subframe. For example, in a TDD configuration with 4 downlink subframes and 1 plink subframe per 5 ms, there are 55·4 combinations, corresponding to more than 46 bits of information.
Currently, there is no PUCCH format in LTE specified capable of carrying such a large number of bits.